1. Field of the Invention
The present invention relates to a process for production of fine and ultrafine zinc powders by electrolysis in a basic medium.
2. Technological Background
The electrolysis of zinc in a caustic medium has been studied for several decades, but its industrial applications have been rare.
Such electrolysis presents some non-negligible advantages:
1. overvoltage of the hydrogen in caustic medium less than in acid medium;
2. production of a very fragile deposit of zinc, eliminating the high costs of scraping of the electrodes necessary in acid medium; and
3. fewer problems of corrosion than in acid medium, allowing lower investment.
The deposit of zinc which is obtained in alkaline medium is slightly coherent, very slightly adherent and furnishes zinc powders hence, for the usual of zinc, the necessity of fusion and of ingoting of the powders is created. In this spirit, the production of zinc in fine particles may be considered as a drawback because high radioactivity leads to an easier oxidation and therefore to a lower yield of fusion (85.90% against 97.99% by the conventional processes).
This phenomenon probably explains the little success of electrolysis in caustic medium.
However, the existence of a large market of zinc powders and dust in the chemistry of catalysts, fillers in general and more particularly for the anti-corrosion paints industry, has guided the study of this type of electrolysis during recent decades.
The zinc powders presently marketed are obtained by thermal process, by fusion and vaporization in vacuo of ingots of zinc. They furnish a fairly wide range of production, from coarse zinc powder to fine and ultra-fine zinc powders.
The powders and dusts obtained by thermal process have the following characteristics:
appearance: matt blue-grey powder PA1 density: 6.9 to 7.1 g/cm.sup.3 PA1 apparent density: 2 to 2.5 g/cm.sup.3 PA1 specific surface area: 1100 to 3300 cm.sup.2 /g. PA1 shape: spherical for the dust, shredded and irregular for the powders d.sub.90 =6 to 17.5.mu. PA1 d.sub.50 =5 to 20.mu. depending on the varieties PA1 oversize at 50.mu. less than 1% PA1 chemical composition: PA1 total Zn: 98.5% PA1 metal Zn: 92 to 96% PA1 Pb=0.15%. PA1 appearance: blue-grey powder with metal lustre PA1 density: 6.9 to 7.1 g/cm.sup.3 PA1 apparent density: 0.9 to 1.5 g/cm.sup.3 PA1 specific surface area: 3000 to 4500 cm.sup.2 /g PA1 shape: dendritic or needle-shaped PA1 d.sub.90 =160 to 325.mu. PA1 d.sub.50 =60 to 90.mu. PA1 oversize at 50.mu.:60 to 70% PA1 chemical composition: PA1 total Zn: 99 to 99.7%-type Z 4-9 PA1 metal Zn: 90 to 97% PA1 Pb: 0.005%. PA1 voltage at the terminals of the cell: U=2.8 to 3.5 V PA1 current density: J=750 to 900 A/m.sup.2 PA1 distance anode/cathode: d=2.0 to 3.8 cm PA1 concentration of caustic soda: C.sub.NaOH =240 g/l PA1 concentration of zinc: C.sub.Zn =5 to 45 g/l PA1 temperature: ambient--19.degree. to 22.degree. C. PA1 1. current density PA1 2. concentration of the electrolyte PA1 3. temperature PA1 4. stirring of the bath PA1 5. presence of colloids PA1 6. presence of metallic complexes.
On the other hand, the powders obtained by alkaline electrolysis have the following characteristics:
The essential differences therefore reside in the higher specific surface area for the alkaline powders, their much coarser granulometry and their very different shapes, namely:
spherical for the powders by thermal process; and
planar for the powders obtained by alkaline electrolysis.
Thus, the powders obtained by alkaline electrolysis must be ground in order to approach the granulometric standards of the powders obtained by thermal process. However, grinding does not make it possible to obtain powders having a d50=20.mu. (compared to d50=7.5.mu. on average for the powders obtained by thermal process).
Hence the research has been conducted, intended to reduce the d50 of the powders obtained by electrolysis in alkaline medium.
The tests were carried out on the basis of the following parameters:
Electroplating obeys two processes:
1. nucleation: formation of blanks of crystals; and PA0 2. growth of the crystal.
The size of the grains obtained depends essentially on the ratio of the speeds of these two processes. If the speed of nucleation is higher than the speed of growth of the crystal, the deposit will be fine, and vice versa. The crystals will grow quickly and will therefore be coarse if the metal ions arriving at the solid-liquid interface are numerous. On the other hand, the centers of nucleation will be abundant and the crystals will grow slowly and will therefore be fine, if the metal ions arriving at the solid-liquid interface are rare.
Hence, in essence, the concentration of the metal ions near the interface, therefore in the NERNST layer, will determine the granulometry of the deposit. On these theoretical bases, the earlier studies have concerned the monitoring of the process of diffusion and therefore the following parameters:
All these techniques are interested in the electrolyte and in the faradic current, but none studies the electrode.
Now, electrolysis causes relationship to intervene between the current, the electrolyte and the electrodes. In the studies on alkaline electrolysis, the study of the cathode has always been seen from the standpoint of corrosion and it has been directed to the search for a corrosion-resistant material.
The essential conclusions of this study are that all the materials more electropositive than zinc and not soluble in caustic solutions may be used to constitute the cathode. In this way, the metals used were stainless steel, magnesium or alloys rich in magnesium (electron metal) and incidentally zinc.
At the beginning of alkaline electrolysis, a very fine film of zinc covers the surface of the cathode; subsequently the zinc is deposited in the form of needles or dendrites.
The mechanical strength and appearance of this film is variable depending on the metal constituting the cathode and depending on the surface appearance of this cathode.
On all the metals, apart from magnesium, this film is very adherent and the recovery of the zinc, by a light scraping, respects it. However, on certain cathodes, blisters and blisterings appear, corresponding to the detachment of this sub-layer. This phenomenon is highly detrimental to the granulometry, as it produces plates of size varying from one millimeter to one centrimeter. Detailed observation shows that, in these zones, very fine irregularities of the surface of the cathode are visible--striae, pitting, . . . --probably prior to the use of the metal as electrode.
After numerous trials directed to the influence of the surface state of the cathode on the granulometry of the zinc powder obtained, it has been discovered, according to the invention, that it was possible to obtain a fine or ultra-fine powder because of a special treatment of the cathode used for electrolysis.